Method and apparatus for extracting ions from an ion source for use in ion implantation

ABSTRACT

Thermal control is provided for an extraction electrode of an ion-beam producing system that prevents formation of deposits and unstable operation and enables use with ions produced from condensable vapors and with ion sources capable of cold and hot operation. Electrical heating of the extraction electrode is employed for extracting decaborane or octadecaborane ions. Active cooling during use with a hot ion source prevents electrode destruction, permitting the extraction electrode to be of heat-conductive and fluorine-resistant aluminum composition. The service lifetime of the system is enhanced by provisions for in-situ etch cleaning of the ion source and extraction electrode, using reactive halogen gases, and by having features that extend the service duration between cleanings, including accurate vapor flow control and accurate focusing of the ion beam optics. A remote plasma source delivers F or Cl ions to the de-energized ion source for the purpose of cleaning deposits in the ion source and the extraction electrode. These techniques enable long equipment uptime when running condensable feed gases such as sublimated vapors, and are particularly applicable for use with so-called cold ion sources and universal ion sources. Methods and apparatus are described which enable long equipment uptime when decaborane and octadecaborane are used as feed materials, as well as when vaporized elemental arsenic and phosphorus are used, and which serve to enhance beam stability during ion implantation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in part of International PatentApplication No. PCT/US2004/041525, filed on Dec. 9, 2004, which, inturn, claims priority to and claims the benefit of U.S. PatentApplication No. 60/529,343, filed on Dec. 12, 2003.

FIELD OF THE INVENTION

The present invention relates to producing ion beams in which one ormore gaseous or vaporized feed materials is ionized in an ion sourcefrom which the ions are extracted by an extraction electrode. It alsorelates to a method and apparatus for operating an ion source andextraction electrode to produce an ion beam for ion implantation ofsemiconductor substrates and substrates for flat panel displays. Inparticular the invention concerns extension of the productive time (i.e.the “uptime”) of systems that produce ion beams and to maintainingstable ion-extraction conditions during the productive time.

BACKGROUND

Ion beams are produced from ions extracted from an ion source. An ionsource typically employs an ionization chamber connected to a highvoltage power supply. The ionization chamber is associated with a sourceof ionizing energy, such as an arc discharge, energetic electrons froman electron-emitting cathode, or a radio frequency or microwave antenna,for example. A source of desired ion species is introduced into theionization chamber as a feed material in gaseous or vaporized form whereit is exposed to the ionizing energy. Extraction of resultant ions fromthe chamber through an extraction aperture is based on the electriccharge of the ions. An extraction electrode is situated outside of theionization chamber, aligned with the extraction aperture, and at avoltage below that of the ionization chamber. The electrode draws theions out, typically forming an ion beam. Depending upon desired use, thebeam of ions may be mass analyzed for establishing mass and energypurity, accelerated, focused and subjected to scanning forces. The beamis then transported to its point of use, for example into a processingchamber. As the result of the precise energy qualities of the ion beam,its ions may be implanted with high accuracy at desired depth intosemiconductor substrates.

The precise qualities of the ion beam can be severely affected bycondensation and deposit of the feed material or of its decompositionproducts on surfaces of the ion beam-producing system, and in particularsurfaces that affect ionization, ion extraction and acceleration.

The Ion Implantation Process

The conventional method of introducing a dopant element into asemiconductor wafer is by introduction of a controlled energy ion beamfor ion implantation. This introduces desired impurity species into thematerial of the semiconductor substrate to form doped (or “impurity”)regions at desired depth. The impurity elements are selected to bondwith the semiconductor material to create electrical carriers, thusaltering the electrical conductivity of the semiconductor material. Theelectrical carriers can either be electrons (generated by N-typedopants) or “holes” (i.e., the absence of an electron), generated byP-type dopants. The concentration of dopant impurities so introduceddetermines the electrical conductivity of the doped region. Many such N-and P-type impurity regions must be created to form transistorstructures, isolation structures and other such electronic structures,which collectively function as a semiconductor device.

To produce an ion beam for ion implantation, a gas or vapor feedmaterial is selected to contain the desired dopant element. The gas orvapor is introduced into the evacuated high voltage ionization chamberwhile energy is introduced to ionize it. This creates ions which containthe dopant element (for example, in silicon the elements As, P, and Sbare donors or N-type dopants, while B and In are acceptors or P-typedopants). An accelerating electric field is provided by the extractionelectrode to extract and accelerate the typically positively chargedions out of the ionization chamber, creating the desired ion beam. Whenhigh purity is required, the beam is transported through mass analysisto select the species to be implanted, as is known in the art. The ionbeam is ultimately transported to a processing chamber for implantationinto the semiconductor wafer.

Similar technology is used in the fabrication of flat-panel displays(FPD's) which incorporate on-substrate driver circuitry to operate thethin-film transistors which populate the displays. The substrate in thiscase is a transparent panel such as glass to which a semiconductor layerhas been applied. Ion sources used in the manufacturing of FPD's aretypically physically large, to create large-area ion beams of boron,phosphorus and arsenic-containing materials, for example, which aredirected into a chamber containing the substrate to be implanted. MostFPD implanters do not mass-analyze the ion beam prior to its reachingthe substrate.

Ion Contamination

In general, ion beams of N-type dopants such as P or As should notcontain any significant portion of P-type dopant ions, and ion beams ofP-type dopants such as B or In should not contain any significantportion of N-type dopant ions. Such a condition is called“cross-contamination” and is undesirable. Cross-contamination can occurwhen source feed materials accumulate in the ion source, and the sourcefeed material is then changed, for example, when first running elementalphosphorus feed material to generate an N-type P⁺ beam, and thenswitching to BF₃ gas to generate a P-type BF₂ ⁺ beam.

A serious contamination effect occurs when feed materials accumulatewithin the ion source so that they interfere with the successfuloperation of the source. Such a condition invariably has called forremoval of the ion source and the extraction electrode for cleaning orreplacement, resulting in an extended “down” time of the entire ionimplantation system, and consequent loss of productivity.

Many ion sources used in ion implanters for device wafer manufacturingare “hot” sources, that is, they operate by sustaining an arc dischargeand generating a dense plasma; the ionization chamber of such a “hot”source can reach an operating temperature of 800 C or higher, in manycases substantially reducing the accumulation of solid deposits. Inaddition, the use of BF₃ in such sources to generate boron-containingion beams further reduces deposits, since in the generation of a BF₃plasma, copious amounts of fluorine ions are generated; fluorine canetch the walls of the ion source, and in particular, recover depositedboron through the chemical production of gaseous BF₃. With other feedmaterials, however, detrimental deposits have formed in hot ion sources.Examples include antimony (Sb) metal, and solid indium (In), the ions ofwhich are used for doping silicon substrates.

Cold ion sources, for example the RF bucket-type ion source which usesan immersed RF antenna to excite the source plasma (see, for example,Leung et al., U.S. Pat. No. 6,094,012, herein incorporated byreference), are used in applications where either the design of the ionsource includes permanent magnets which must be kept below their Curietemperature, or the ion source is designed to use thermally-sensitivefeed materials which break down if exposed to hot surfaces, or whereboth of these conditions exist. Cold ion sources suffer more from thedeposition of feed materials than do hot sources. The use of halogenatedfeed materials for producing dopants may help reduce deposits to someextent, however, in certain cases, non-halogen feed materials such ashydrides are preferred over halogenated compounds. For non-halogenapplications, ion source feed materials such as gaseous B₂H₆, AsH₃, andPH₃ are used. In some cases, elemental As and P are used, in vaporizedform. The use of these gases and vapors in cold ion sources has resultedin significant materials deposition and has required the ion source tobe removed and cleaned, sometimes frequently. Cold ion sources which useB₂H₆ and PH₃ are in common use today in FPD implantation tools. Theseion sources suffer from cross-contamination (between N- and P-typedopants) and also from particle formation due to the presence ofdeposits. When transported to the substrate, particles negatively impactyield. Cross-contamination effects have historically forced FPDmanufacturers to use dedicated ion implanters, one for N-type ions, andone for P-type ions, which has severely affected cost of ownership.

Borohydrides

Borohydride materials such as B₁₀H₁₄ (decaborane) and B₁₈H₂₂(octadecaborane) have attracted interest as ion implantation sourcematerials. Under the right conditions, these materials form the ionsB₁₀H_(x) ⁺, B₁₀H_(x) ⁻, B₁₈H_(x) ⁺, and B₁₈H_(x) ⁻. When implanted,these ions enable very shallow, high dose P-type implants for shallowjunction formation in CMOS manufacturing. Since these materials aresolid at room temperature, they must be vaporized and the vaporintroduced into the ion source for ionization. They are low-temperaturematerials (e.g., decaborane melts at 100 C, and has a vapor pressure ofapproximately 0.2 Torr at room temperature; also, decaborane dissociatesabove 350 C), and hence must be used in a cold ion source. They arefragile molecules which are easily dissociated, for example, in hotplasma sources.

Contamination Issues of Borohydrides

Boron hydrides such as decaborane and octadecaborane present a severedeposition problem when used to produce ion beams, due to theirpropensity for readily dissociating within the ion source. Use of thesematerials in Bernas-style arc discharge ion sources and also inelectron-impact (“soft”) ionization sources, have confirmed thatboron-containing deposits accumulate within the ion sources at asubstantial rate. Indeed, up to half of the borohydride vapor introducedinto the source may stay in the ion source as dissociated, condensedmaterial. Eventually, depending on the design of the ion source, thebuildup of condensed material interferes with the operation of thesource and necessitates removal and cleaning of the ion source.

Contamination of the extraction electrode has also been a problem whenusing these materials. Both direct ion beam strike and condensed vaporcan form layers that degrade operation of the ion beam formation optics,since these boron-containing layers appear to be electricallyinsulating. Once an electrically insulating layer is deposited, itaccumulates electrical charge and creates vacuum discharges, orso-called “glitches”, upon breakdown. Such instabilities affect theprecision quality of the ion beam and can contribute to the creation ofcontaminating particles.

SUMMARY OF THE INVENTION

Objects of this invention are to provide a method and apparatus forproducing ions beams without disturbance in the stability of the ionbeam by electric discharges at the extraction electrode and to provide amethod and apparatus for producing an ion beam which increases servicelifetime and reduces equipment down time.

The invention features an extraction electrode for extracting ions fromthe ion source in which the electrode includes an active thermal controlsystem.

The invention also features in-situ cleaning procedures and apparatusfor an ion source and associated extraction electrodes and similarcomponents of the ion-beam producing system, which periodicallychemically remove deposits, increasing service lifetime and performance,without the need to disassemble the system.

The invention also features an actively heated ion extraction electrodewhich consists of a material which reduces the frequency and occurrenceof electrical discharges, preferably this material being a metal.

Another feature is, in general, heating an extraction electrode abovethe condensation temperature of feed materials to an ion source, inpreferred cases the electrode being comprised of metal, preferablyaluminum or molybdenum.

The invention also features an ion extraction electrode comprised ofaluminum, suitable for in situ reactive gas cleaning. Preferredembodiments include provisions for active temperature control of theextraction electrode adapted to the type of ion source with which theelectrode is constructed to operate. Embodiments feature active heatingof the extraction electrode for operation with cool-operating ionsources, active cooling of the extraction electrode for operation withhot-operating ion sources, and both active heating and cooling of theextraction electrode for selective operation with cool and hot-operatingion sources.

These and other innovations of the invention may include one or more ofthe following features:

A supply of a reactive gas is provided and introduced into the ionsource, and the ion source and extraction electrode are cleaned in situthrough exposure to reactive products from that supply such as atomicfluorine, F, or molecular fluorine, F₂; the atomic or molecular fluorineis injected into the ion source from a remote plasma source; the ionsource and extraction electrode are cleaned through exposure to gaseousClF₃ flowing from a remote supply; reactive components of the ionizationapparatus are shielded from reactive gas during the cleaning phase ofoperation; the ion source is fabricated of aluminum; the extractionelectrode is fabricated of aluminum; the front face of the extractionelectrode is devoid of sharp or rough features; the plates of theextraction electrode are actively temperature controlled; the plates ofthe extraction electrode are actively heated; heating of the extractionelectrode is radiative or is resistive; the plates of the extractionelectrode in other situations are actively cooled.

Another feature is the use of the features described with apparatussuitable to form “cluster” or “molecular” ion beams, of feed materialthat is particularly subject to thermal breakdown and deposit.

While most ion implantation experts would agree that the use ofborohydrides to form “cluster” ion beams such as B₁₀H_(x) ⁺ and B₁₈H_(x)⁺ is very attractive for shallow junction formation, means to ionize andtransport these large molecules have presented problems. For example,U.S. Pat. Nos. 6,288,403 and 6,452,338 describe ion sources which havesuccessfully produced decaborane ion beams. However, such decaborane ionsources have been found to exhibit particularly short service life ascompared to other commercial ion sources used in ion implantation. Thisshort service life has been primarily due to the accumulation ofboron-containing deposits within the ion source, and the deposition ofinsulating coatings on the ion extraction electrode, which has lead tobeam instabilities requiring implanter shut down and maintenance.

According to another feature, means are provided to substantially reducethe deposition of such deposits in the borohydride ion source and on theion extraction electrode, and means are provided to clean deposits onthese components without removing them from the ion implanter, i.e.,in-situ. This invention enables the commercial use of borohydridecluster beams in semiconductor manufacturing with long service lifetime.

A particular aspect of the invention is a system for generating an ionbeam comprising an ion source in combination with an activelytemperature-controlled extraction electrode and a reactive gas cleaningsystem, the ion source comprising an ionization chamber connected to ahigh voltage power supply and having an inlet for gaseous or vaporizedfeed materials, an energizeable ionizing system for ionizing the feedmaterial within the ionization chamber and an extraction aperture thatcommunicates with a vacuum housing, the vacuum housing evacuated by avacuum pumping system, the extraction electrode disposed in the vacuumhousing outside of the ionization chamber, aligned with the extractionaperture of the ionization chamber and adapted to be maintained at avoltage below that of the ionization chamber to extract ions through theaperture from within the ionization chamber, and the reactive gascleaning system operable when the ionization chamber and ionizing systemare de-energized to provide a flow of reactive gas through theionization chamber and through the ion extraction aperture to react withand remove deposits on at least some of the surfaces of the iongenerating system.

Preferred embodiments of this aspect have one or more of the followingfeatures.

The system is constructed for use in implanting ions in semiconductorwafers, the ionization chamber having a volume less than about 100 mland an internal surface area of less than about 200 cm².

The system is constructed to produce a flow of the reactive gas into theionization chamber at a flow rate of less than about 2 Standard LitersPer Minute.

The extraction electrode is constructed to produce a beam of acceleratedions suitable for transport to a point of utilization.

The extraction electrode is located within a path of reactive gas movingfrom the extraction aperture to the vacuum pumping system so that theextraction electrode is cleaned by the reactive gas.

The extraction electrode is associated with a heater to maintain theelectrode at elevated temperature during extraction by the extractionelectrode of ions produced in the ionization chamber, e.g. above thecondensation temperature, below the disassociation temperature, ofsolid-derived, thermally sensitive vapors.

The extraction electrode is associated with a cooling device, e.g. whenthe electrode is formed of thermally sensitive material and is used witha hot ion source.

The extraction electrode has a smooth, featureless aspect.

The reactive gas cleaning system comprises a plasma chamber, the plasmachamber arranged to receive a feed gas capable of being disassociated byplasma to produce a flow of reactive gas through a chamber outlet, and aconduit for transporting the reactive gas to the ionization chamber.

The plasma chamber is constructed and arranged to receive anddisassociate a compound capable of being disassociated to atomicfluorine, for instance NF₃, C₃F₈ or CF₄.

The reactive gas cleaning system is constructed and arranged to share aservice facility associated with the ion source.

The system is constructed to direct an ion beam through a mass analyzer,in which the reactive gas cleaning system is constructed and arranged toshare a service facility with the mass analyzer.

The reactive gas cleaning system comprises a conduit from a container ofpressurized reactive gas, for instance ClF₃.

The system is in combination with an end-point detection system adaptedto at least assist in detecting substantial completion of reaction ofthe reactive gas with contamination on a surface of the system forgenerating an ion beam.

The end point detection system comprises an analysis system for thechemical makeup of gas that has been exposed to the surface duringoperation of the reactive gas cleaning system.

A temperature detector is arranged to detect substantial termination ofan exothermic reaction of the reactive gas with contamination on asurface of the system.

The energizeable ionizing system includes a component within or incommunication with the ionization chamber that is susceptible to harm bythe reactive gas and means are provided to shield the component fromreactive gas flowing through the system.

The means to shield the component comprises an arrangement for producinga flow of inert gas, such as argon, past the component.

The means for shielding a component comprises a shield member that isimpermeable to the reactive gas.

The system is constructed to operate with reactive halogen gas as thereactive gas and the extraction electrode and associated parts comprisealuminum (Al) or alumina (Al₂O₃).

The ion source is constructed to produce ions within the ionizationchamber via an arc-discharge, an RF field, a microwave field or anelectron beam.

The system is associated with a vaporizer of condensable solid feedmaterial for producing feed vapor to the ionization chamber.

The ion source is constructed to vaporize feed material capable ofproducing cluster or molecular ions, and the ionization system isconstructed to ionize the material to form cluster or molecular ions forimplantation.

The vacuum housing of the system is associated with a pumping systemcomprising a high vacuum pump capable of producing high vacuum and abacking pump capable of producing vacuum, the high vacuum pump operableduring operation of the ion source, and being capable of being isolatedfrom the vacuum housing during operation of the reactive cleaningsystem, the backing pump operable during operation of the reactive gascleaning system.

The system is associated with an ion implantation apparatus, theapparatus constructed to transport ions following the extractionelectrode implantation station within a vacuum chamber. In preferredembodiments an isolation valve is included for isolating theimplantation station from the ionization chamber and the extractionelectrode during operation of the reactive gas cleaning system.

The ion source is constructed and adapted to generate dopant ions forsemiconductor processing, and the reactive gas cleaning system isadapted to deliver fluorine, F, or chlorine, Cl, ions to the ionizationchamber or the extraction electrode for cleaning deposits from asurface.

The ion source is adapted to be temperature-controlled to a giventemperature.

The ion source is adapted to generate a boron-containing ion beam; inpreferred embodiments the boron-containing ion beam is generated byfeeding vaporized borohydride material into the ion source, especiallyeither decaborane, B₁₀H₁₄ or octadecaborane, B₁₈H₂₂.

The ion source is adapted to generate arsenic-containing ion beams.

The ion source is adapted to generate phosphorus-containing ion beams.

The ionization chamber of the ion source comprises aluminum.

The ionization chamber of the ion source or the extraction electrodecomprises a material resistant to attack by halogen gases such asfluorine, F.

Another particular aspect of the invention is a method of in-situcleaning using the system of any of the foregoing description, or of anion source and temperature-controlled ion extraction electrodeassociated with an ion implanter, in which reactive halogen gas isflowed into an ion source while the ion source and ion extractionelectrode are de-energized and under vacuum.

Embodiments of this aspect have one or more of the following features.

The reactive halogen gas is fluorine, F.

The reactive halogen gas is chlorine, Cl.

The fluorine gas is introduced into the ion source from a remote plasmasource.

The fluorine gas is produced in the remote plasma source by an NF₃plasma.

The fluorine gas is produced in the remote plasma source by a C₃F₈ orCF₄ plasma.

The reactive halogen gas is ClF₃.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized decaborane, B₁₀H₁₄.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized octadecaborane, B₁₈H₂₂.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized arsenic-containing compounds, such as arsine, AsH₃,or elemental arsenic, As.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized phosphorus-containing compounds, such as elementalphosphorus, P, or phosphine, PH₃.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized antimony-containing compounds, such astrimethylantimony, Sb(CH₄)₃, or antimony pentaflouride, SbF₅.

The cleaning procedure is conducted for an ion source in situ in an ionimplanter between changing ion source feed materials in order to implanta different ion species.

Another particular aspect of the invention is an ion implantation systemhaving an ion source and an extraction electrode for extracting ionsfrom the ion source, in which the extraction electrode includes a heaterconstructed to maintain the electrode at an elevated temperaturesufficient to substantially reduce condensation on the electrode ofgases or vapors being ionized and products produced therefrom. Anotheraspect is such an extraction electrode, per se, useful in such system.

Embodiments of these aspects have one or more of the following features.

The electrode comprises aluminum.

The electrode comprises molybdenum.

The electrode is heated by radiative heating.

The electrode is heated by a resistive heating element).

The temperature of the electrode is controlled to a desired temperature;in embodiments the temperature is between 150 C and 250 C.

The electrode is periodically cleaned in situ by exposure to reactivehalogen-containing gas.

The electrode comprises at least two electrode elements constructed andarranged in close succession along a beam path from the ion source, theelectrode elements having elongated, slot-form beam apertures throughwhich a ribbon-like ion beam passes, the heater including heaterportions disposed on each of the long sides of the slot-form apertures.In some preferred forms at least one electrode element comprises aninner portion defining its beam aperture and an outer portion in heatconductive relation to the inner portion, the outer portion defining aheat receptor face for absorbing radiated heat. In some preferred forms,at least one of the electrode elements comprises a portion that definesits beam aperture, this portion being exposed to be a receptor forabsorbing radiated heat.

The heater comprises a continuous electrical resistance heating element.In preferred forms this heating element is arranged to heat multipleelectrode elements by radiative heating. Preferably the heating elementis sealed within a protective tube to form a tubular heater, the tubeconstructed to be heated internally by the heating element and the tubeexposed to heat the electrode elements by radiative heating, andpreferably the tube being of circular configuration and disposed tosurround beam-path defining portions of the electrode elements.

In preferred forms an electrode element comprises an inner portiondefining its beam aperture and an outer portion in heat conductiverelation to the inner portion, the outer portion defining a heatreceptor face for absorbing radiated heat, a tubular heater surroundingthe beam path and opposed to the receptor face in radiant heatingrelationship. In one preferred form, a pair of electrode elements of theextraction electrode each comprises an inner portion defining its beamaperture and an outer portion in heat conductive relation to the innerportion, the outer portions defining heat receptor faces for absorbingradiated heat, the tubular heater disposed between, and in radiantheating relationship to the receptor faces of these two electrodeelements. This arrangement may be employed in a two-electrode elementconfiguration or in a configuration having more than two electrodeelements. In one preferred form, besides the pair of electrode elementsthere is at least a third electrode element disposed between the pair,the third electrode element comprising a portion that defines its beamaperture, this portion exposed to be a heat receptor for heat radiatingradially inwardly from the surrounding tubular heater.

Another aspect of the invention is a method of in-situ cleaning of anion extraction electrode of any of the systems described or atemperature-controlled ion extraction electrode which is associated withan ion implanter, in which reactive halogen gas is flowed over the ionextraction electrode while the electrode is in situ and under vacuum.Another aspect is a temperature-controlled ion extraction electrodeconstructed for in situ cleaning by such gas.

Embodiments of this aspect have one ore more of the following features.

The reactive halogen gas is fluorine, F or chlorine, Cl.

Fluorine gas is introduced from a remote plasma source into a vacuumhousing in which the extraction electrode resides.

Fluorine gas is produced in the remote plasma source by a NF₃ plasma.

Fluorine gas is produced in the remote plasma source by a C₃F₈ or CF₄plasma.

The reactive gas is ClF₃.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized decaborane, B₁₀H₁₄.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized octadecaborane, B₁₈H₂₂.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized arsenic-containing compounds, such as arsine, AsH₃,or elemental arsenic, As.

The cleaning procedure is conducted to remove deposits after the ionsource has ionized phosphorus-containing compounds, such as elementalphosphorus, P, or phosphine, PH₃.

The cleaning procedure is conducted between changing ion source feedmaterials in order to implant a different ion species.

Ion Source and Ion Extraction Electrode Provided with In-Situ EtchCleaning

According to a preferred embodiment, the in situ chemical cleaningprocess utilizes atomic F gas, to effectively clean deposits from theion source and from the ion extraction electrode, while the ion sourceand extraction electrode remain installed in the ion beam-producingsystem. In a preferred embodiment an electron impact ion source withcooled chamber walls is employed. Preferably, the ionization chamber andsource block and the extraction electrode, comprise aluminum, i.e. arefabricated of aluminum or of an aluminum containing alloy, enablingaluminum fluoride to be created on the aluminum surfaces to act as apassivating layer, that prevents further chemical attack by F.Insulators of the assembly are preferably formed of alumina (Al₂O₃)which is also resistant to attack by F.

One embodiment of this feature uses the outlet of a remote reactive gassource directly coupled to an inlet to the ion source.

In a preferred embodiment the reactive gas source is a plasma sourcewhich introduces an etch feed gas, such as NF₃ or C₃F₈, into asupplemental ionization chamber. By sustaining a plasma in thesupplemental chamber, reactive gases such as F and F₂ are produced, andthese reactive gases, introduced to the main ion source, chemicallyattack the deposited materials. By-products released in the gas phaseare drawn through the extraction aperture of the ionization chamber,past the extraction electrode, and are pumped away by the vacuum systemof the installation, cleaning the chamber and the ion extractionelectrode.

Deposition Model

It is a generally observed principle of physics that when two objectsinteract, there can be more than one outcome. Furthermore, one canassign probabilities or likelihoods to each outcome such that, when allpossible outcomes are considered, the sum of their individualprobabilities is 100%. In atomic and molecular physics such possibleoutcomes are sometimes called “channels” and the probability associatedwith each interaction channel is called a “cross section”. Moreprecisely, the likelihood of two particles (say, an electron and a gasmolecule) interacting with each other at all is the “total crosssection” while the likelihoods of certain types of interactions (such asthe interaction represented by the electron attaching itself to the gasmolecule thus forming a negative ion, or by removing an electron fromthe gas molecule thus forming a positive ion, or by dissociating themolecule into fragments, or by elastically scattering from the moleculewith no chemical change of the molecule) are the “partial crosssections”.

This state of affairs can be represented by a mathematical relationwhich expresses the total cross section σ_(T) as the sum of its ipartial cross sections:σ_(T)=σ₁+σ₂+σ₃+ . . . σ_(i), or  (1)σ_(T)=Σσ_(i).  (2)The ion sources used in ion implanters typically display modestionization fractions. That is, only a small fraction (from a few percentto a few tens of percent) of the gas or vapor fed into the ion source isionized. The rest of the gas or vapor typically leaves the source in thegas phase, either in its original state or in some other neutral state.That is, the ionization cross section is much smaller than the totalcross section. Of course, some of the gas components can stay in the ionsource as deposited materials, although this tends to be a smallpercentage of the total for the commonly used implantation feedmaterials. While feed materials vaporized by heating such as elementalAs or P more readily produce deposits than do normally gaseous feedmaterials, the heated vapor tends to stay in the gas phase if the wallsof the ion source are at a higher temperature than the vaporizer, and donot pose a severe deposition risk. However, significant detrimentaldeposits may still be produced when producing boron beams from gaseousBF3 feed gas, for example, as well as beam from In and Sb.

Also, in general, over time, deposits of condensable materials do occuron the extraction electrode and on certain other components of ionproducing systems, affecting their operational life before disassemblyand cleaning.

Furthermore, in the case of the borohydrides, the total cross sectionrepresenting all interactions with the ionizing medium (i.e., electronsin the ion source) appears large, the ionization cross section is small,and by far the largest cross section represents the channel fordissociation of the borohydride molecules into non-volatile fragments,which then remain on surfaces in the ion source. The problem ofdeposition of these fragments is adversely influenced by cooling of theionization chamber walls in an effort to reduce thermal decomposition ofthe feed material. In sum, it appears that deposition from borohydridesof boron-containing fragments in the source is a fundamental phenomenonwhich would be observed in any type of ion source acting on thismaterial, and solution to the problem is of broad, critical interest tothe semiconductor manufacturing industry. It is also found thatcontamination of the ion extraction electrode with insulative depositsis a problem with borohydrides, as described more fully below.

Electron Impact Ion Source Suitable for Borohydrides

An ion source particularly suitable for borohydrides is anelectron-impact ion source which is fully temperature-controlled (seeU.S. Pat. Nos. 6,452,338 and 6,686,595; also International Applicationno. PCT/US03/20197, each herein incorporated by reference); also seeFIG. 7. Instead of striking an arc-discharge plasma to create ions, theion source uses a “soft” electron-impact ionization of the process gasby energetic electrons injected in the form of one or more focusedelectron beams. This “soft” ionization process preserves these largemolecules so that ionized clusters are formed. As seen in FIG. 7, solidborohydride material is heated in a vaporizer and the vapor flowsthrough a vapor conduit to a metal chamber, i.e., the ionizationchamber. An electron gun located external to the ionization chamberdelivers a high-current stream of energetic electrons into theionization chamber; this electron stream is directed roughly paralleland adjacent to an extended slot in the front of the chamber. Ions areextracted from this slot by an ion extraction electrode, forming anenergetic ion beam. During transport of the sublimated borohydride vaporto the ionization chamber all surfaces are held at a higher temperaturethan that of the vaporizer (but well below the temperature ofdissociation), to prevent condensation of the vapor. Many hours oftesting have confirmed that the surfaces of the vapor feed and valvesindeed remain clean when such temperature control is implemented.

Extension of Ion Source Lifetime with Decaborane Between Cleanings

The impact of vapor flow rate on source lifetime (maintenance interval)was studied in a quantitative manner. The electron impact ion source wasrun continuously with decaborane feed material under controlledconditions at a given vapor flow, until it was determined that thebuildup of material was causing a significant decrease in decaboranebeam current. Five different flow rates were tested, ranging from about0.40 sccm to 1.2 sccm. This resulted in mass-analyzed decaborane beamcurrents (B₁₀H_(x) ⁺) ranging from about 150 μA to 700 μA. It is notedthat typical feed gas flows in ion sources used in ion implantationrange from 1 to about 3 sccm, so this test range is considered a “low”flow regime.

The results of these lifetime tests are summarized in FIG. 5. Itsuggests a simple model, a hyperbolic function. This is not unexpected;if one assumes zero vapor flow, then the source lifetime would, inprinciple, diverge; and if one assumes very high vapor flow, then sourcelifetime would decrease asymptotically to zero. Thus, the model can beexpressed by:(flow rate)×(flow duration)=constant.  (3)

Equation (3) simply states that lifetime (i.e., flow duration) isinversely proportional to flow rate; the constant is the amount ofdeposited material. If equation (3) is accurate, then the fraction ofdeposited material is independent of the rate of flow of material, whichis consistent with our model describing a fixed cross section fordissociation and subsequent deposition. These data show that, using theelectron impact ion source with about 0.5 sccm decaborane vapor flow,dedicated decaborane operation can be sustained for more than 100 hours.While this is acceptable in many cases, in commercial semiconductorfabrication facilities, source lifetimes of well over 200 hours aredesired. When the ion source is used in conjunction with the novelin-situ cleaning procedure of the present invention, greatly extendedsource lifetimes are achieved. The in situ cleaning includes cleaningthe ion extraction electrode assembly as has been fully described below.

Advantages of Certain Features of In-Situ Ion Source and Ion ExtractionElectrode Chemical Cleaning

There are several very important advantages to using a supplemental ionsource to produce the reactive gas for in situ cleaning of the ionsource and the ion extraction electrode. Such plasma sources have beendeveloped for effluent removal applications from process exhaust systems(such as the Litmus 1501 offered by Advanced Energy, Inc.), and forcleaning large CVD process chambers (such as the MKS Astron reactive gasgenerator), but to the inventors' knowledge it has not been previouslyrecognized that a remote reactive gas generator could be usefullyapplied to in situ cleaning the ionization chamber of an ion source andthe extraction electrode used to generate an ion beam. Remote reactivegas generators such as the MKS Astron have been used to clean processchambers (i.e., relatively large vacuum chambers wherein semiconductorwafers are processed), an application which uses high flows of feed gas(several Standard Liters per Minute (SLM)), and high RF power applied tothe plasma source (about 6 kW). The system of the present invention canemploy a much more modest feed gas rate, e.g. less than about 0.5 SLM ofNF₃, and much less RF power (less than about 2.5 kW), for the very smallvolume of the ionization chamber of the ion source being cleaned (thevolume of the ionization chamber for an implanter of semiconductorwafers is typically less than about 100 ml, e.g. only about 75 ml, witha surface area of less than about 200 cm², e.g. about 100 cm²). Thereactive gas flow into the ionization chamber is less than about 2Standard Liters Per Minute.

One might think it strange to use an external ion source to generateplasma by-products to introduce into the main ion source of the system;why not just introduce the (e.g., NF₃) gas directly into the main ionsource to create the plasma by-products within that source directly? Thereasons seem not obvious. In order to achieve etch rates which farexceed deposition rates from the feed gas during a small fraction of theuptime (productive period) of the ion implantation system, it is foundthat the reactive gas must be produced and introduced at relatively veryhigh flows into the small ionization chamber, e.g., flows on the orderof 10² to 10³ sccm, compared to typical feed flow rates for the main ionsource for ion implantation in the range of 1-3 sccm; such high flowswould raise the pressure within the ionization chamber of the main ionsource far beyond that for which it is designed to operate for ionimplantation. Furthermore, sustaining a high-density NF₃ plasma withinthe main ion source would etch away sensitive components, such as hottungsten filaments. This is because halogen gases etch refractory metalsat a high rate which increases exponentially with temperature. (Forexample, Rosner et al. propose a model for F etching of a tungstensubstrate:Rate(microns/min)=2.92×10⁻¹⁴ T ^(1/2) N _(F) e ^(−3900/T),  (4)Where N_(F) is the concentration of fluorine in atoms per cm³, and T isthe substrate temperature in degrees Kelvin.)

Since virtually all ion sources for ion implantation incorporate hotfilaments, and since in many cases the ion source chambers are also madeof refractory metals such as Mo and W, or graphite (which isaggressively attacked by F), these ion sources would quickly fail underhigh temperature operating conditions, making the etch cleaning processunusable.

In the presently preferred embodiment, atomic fluorine is caused toenter the cold ionization chamber of the de-energized main ion source ata flow rate of 100 sccm or more, and the total gas flow into theionization chamber is 500 sccm or more. Under these conditions, the gaspressure within the ionization chamber is about 0.5 Torr, while thepressure within the vacuum source housing of the implanter is a few tensof milliTorr or more. In a preferred mode of operation, preceding thecleaning phase, an isolation valve is closed between the vacuum housingof the ion source and the implanter vacuum system, and theturbo-molecular pump of the ion source is isolated. The housing of theion source, including the space containing the ion extraction electrode,is then pumped with high-capacity roughing pumps of the vacuum system(i.e., the pumps which normally back the turbomolecular pumps andevacuate the vacuum system down to a “rough” vacuum).

A different embodiment of a related etch clean process, shown in FIG. 5,utilizes a “dry etch” gas such as ClF₃. As has previously been observed,the ClF₃ molecule breaks up on contact with deposited surfaces to becleaned; thus, atomic fluorine and chlorine are released withoutrequiring the generation of a plasma. While handling of ClF₃ gasrequires special equipment due to its highly reactive nature, it inprinciple can simplify the chemical cleaning process to the extent ofnot requiring an ancillary reactive gas plasma source. Since toxic gasesare routinely fed into an ion source for an ion implanter, much of theequipment is already constructed to be “toxic-gas-friendly”, and aseparate gas distribution system incorporating ClF₃ can be added in astraightforward manner.

Advantages of the in-situ chemical cleaning of the ion source and ionextraction electrode for an ion implanter include: a) extending sourcelife to hundreds, or possibly thousands, of hours before service isrequired; b) reducing or eliminating cross-contamination brought aboutby a species change, for example, when switching from octadecaborane ionimplantation to arsenic or phosphorus ion implantation, and from arsenicor phosphorus ion implantation to octadecaborane ion implantation; andc) sustaining peak ion source performance during the service life of theion source.

For example, performing a 10 minute chemical cleaning protocol everyeight hours (i.e., once every shift change of operating personnel) andbetween each species change would have a minimal impact on the uptime ofthe implanter, and would be acceptable to a modern semiconductorfabrication facility.

Endpoint Detection

It is realized to be beneficial to provide endpoint detection during thecleaning process, so that quantitative information on the efficacy andrequired duration of the cleaning process may be generated, and thereproducibility of the chemical cleaning process may be assured. FIG. 3shows a differentially-pumped quadrupole mass analyzer (Residual GasAnalyzer, RGA) sampling the cleaning process. By monitoring theconcentrations of cleaning gas products such as F, Cl, BF₃, PF₃, AsF₃,AlF₃, WF₆, for example, the cleaning process may be tuned and verified.Alternatively, optical means of monitoring the process may be utilized.An FTIR optical spectrometer can monitor the gases resident in thevacuum housing of the ion source of the implanter, through a viewport.This non-invasive (ex-situ) means to identify chemical species may bepreferable to in-situ monitoring devices in certain cases.Alternatively, see FIG. 4, an extractive FTIR spectrometer may becoupled to the source vacuum housing for endpoint monitoring. A novelmeans to accomplish endpoint detection consists of monitoring thetemperature of the ionization chamber during cleaning. Since thechemical reaction is exothermic, energy is released during the reaction,elevating the chamber temperature. This effect can in principle be usedto establish when the reaction rate is diminished.

Novel Ion Extraction Electrode

Borohydrides such as decaborane and octadecaborane are thermallysensitive materials. They vaporize and condense at temperatures between20 C and 100 C. It is therefore important to maintain all surfaces withwhich these materials come into contact at a temperature higher than thevaporizer temperature (but below the temperature at which theydissociate), to prevent condensation. We have found that contaminationof the extraction electrode is a problem when using such a borohydride.Both direct ion beam strike and condensed feed vapor or products of itsmolecular disassociation can degrade operation of the ion beam formationoptics, since these boron-containing layers appear to be electricallyinsulating. Once electrically insulating layers are deposited, theyacquire an electrical charge (“charge up”) and create vacuum discharges,or “glitches”, upon electrical breakdown. Such a discharge createsinstabilities in the ion beam current and can contribute to the creationof particles that may reach a process chamber to which the ion beam isdirected. An ion implanter which has an ion beam-producing system thatexperiences many glitches per hour is not considered production-worthyin modern semiconductor fabrication facilities. Furthermore, even inabsence of such discharges, as insulating coatings become thicker, theelectric charge on electrode surfaces create unwanted stray electricfields which can result in beam steering effects, creating beam loss andmay adversely affect ion beam quality.

Discovery of new information has led to a robust solution to thisproblem. Most implanter ion extraction electrodes are made of graphite.Graphite has been seen to have many advantages in this application,including low materials cost, ease of machining, high electricalconductivity, low coefficient of thermal expansion, and good mechanicalstability at high temperatures. However, using a graphite extractionelectrode, instabilities were observed after producing an ion beam ofborohydrides. It was suspected that the surfaces of the electrode hadbecome insulating. Samples of the electrode deposits were collected anda chemical analysis performed by x-ray fluorescence spectroscopy. Thestudy revealed a chemical stoichiometry consistent with a boron-carboncompound of the form B₂C, which was found to be insulating. In addition,it appeared that metal surfaces in the vicinity of the ion source,including the front plate (i.e., the ion extraction aperture plate) ofthe ion source also had deposited insulating coatings after long use. Itwas conceived to fabricate the electrode of aluminum, and provideradiant heaters to keep the electrode plates, i.e., the suppression andground electrodes, at a well-controlled, elevated temperature (see FIGS.9, 11) sufficiently high to prevent condensation of decaborane andoctadecaborane. In addition, the suppression electrode, which faces theion source, was fabricated of a single machined piece of aluminum, witha smooth, featureless aspect and all fasteners were located at thebackside of the plates. This configuration dramatically reduced thenumber and severity of discharge points in the event that insulatingcoatings were formed, employing the principle that the electric fieldstress at a “point”, or sharp feature, is many times greater than at asmooth surface.

The extraction electrode, thus produced, demonstrated excellentperformance, and operated reliably for more than 100 hours (at least tentimes as long as the graphite electrode) with very low glitch frequency.This great improvement is attributed to: i) Al construction (i.e., metalversus graphite), ii) Active heating and temperature control of theelectrode plates, and iii) smooth electrode surfaces. It was found thatoperating the electrode plates at 200 C gave good results when runningdecaborane, significantly reducing the amount of deposited material. Ingeneral, the temperature of the extraction electrode should be keptbelow the dissociation temperature of the feed material. In the case ofdecaborane the temperature should be kept below 350 C, preferably in therange 150 C to 250 C. For octadecaborane operation, the temperatureshould not exceed 160 C, since chemical changes occur in octadecaboraneabove this temperature; when running octadecaborane, an extractionelectrode temperature between 120 C and 150 C yields good results.

The radiative design shown in FIG. 11 demonstrated very good temperatureuniformity. Incorporating resistive heaters, particularly using analuminum electrode as illustrated in FIG. 12, can also yield gooduniformity and results in a more compact design requiring lessmaintenance. A further design that incorporates desirable features fromboth of the designs of FIGS. 11 and 12 is described with reference toFIGS. 1A-1L.

For constructing a heated extraction electrode, other metals would alsowork, for example molybdenum. Molybdenum has the advantage of beingrefractory, so it can withstand very high temperatures. It also has goodthermal conductivity. Aluminum, on the other hand, is a column IIIelement like In and B of the periodic table, and therefore offers theadvantage of being only a mild contaminant in silicon (it is a P-typedopant in silicon), while transition metals such as molybdenum are verydetrimental to carrier lifetimes in integrated circuits. Aluminum isalso not readily attacked by halogens, whereas transition metals such asmolybdenum are susceptible to attack, particularly at elevatedtemperatures. The primary disadvantage of aluminum, however, is that itis not a high temperature material, and should be used below about 400C.

For these reasons, depending upon the particular use, the heatedelectrode is constructed of a selected heat-resistant material, aluminumor an aluminum containing alloy often being preferred when used inassociation with in situ etch cleaning.

By providing the alternative of active electrode cooling as well asactive heating, a temperature-controlled ion extraction electrodecomprised of aluminum, suitable for halogen cleaning, may be used withdifferent types of interchangeable ion sources, or with a multi-mode ionsource. The aluminum electrode can be used with cool ion sources (duringwhich the extraction electrode is heated to deter contamination, andavoid unstable operation), and with hot ion sources (during which theextraction electrode is cooled to keep its temperature below about 400C, to maintain its dimensional stability.)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Ion beam generation system incorporating reactive gas cleaning.

FIG. 1A: Ion-extraction electrode assembly useful in the system of FIG.1.

FIGS. 1B and 1C: Cross-sections taken on lines 1B and 1C, respectively,on FIG. 1A;

FIG. 1D: exploded view of the assembly;

FIGS. 1E and 1F: perspective views of the assembly mounted on amanipulator.

FIGS. 1G, 1H and 1I: Orthogonal views of heater of the assembly of FIG.1A;

FIGS. 1J and 1K: cut away and side views of the end portion of theheater;

FIG. 1L: Heater control circuit.

FIG. 2: Second embodiment of ion beam generation system incorporatingreactive gas cleaning.

FIG. 3: Ion beam generation system similar to FIG. 1 but incorporating avaporizer and certain gas distribution elements.

FIG. 4: Ion beam generation system similar to FIG. 2 but incorporating avaporizer and certain gas distribution elements.

FIG. 5: Ion generation system incorporating reactive gas cleaning by theintroduction of ClF₃.

FIG. 6: Gas box for an ion implanter which includes a reactive gasplasma source, feed vapor source, ion source electronics, and facilitiesfor the plasma source.

FIG. 6A: View similar to FIG. 6, showing a vapor flow control system.

FIG. 6B: Valve schematic for an ion beam generating system.

FIG. 7: Electron-impact ion source.

FIG. 7A: Magnified view of a portion of FIG. 7, showing shielding ofelements.

FIG. 7B: Control diagram for an embodiment.

FIG. 8: Ion extraction electrode.

FIG. 9: Ion extraction electrode optics.

FIG. 9A: B₁₈H_(x) ⁺ beam profiles.

FIG. 9B: Extraction electrode having heating and cooling features andtwo electrode elements.

FIG. 9C: Extraction electrode assembly having heating and coolingfeatures and three electrode elements.

FIG. 10: Extraction electrode and manipulator.

FIG. 11: Electrode head-exploded view.

FIG. 12: Second embodiment of electrode head.

FIG. 13: B₁₀H_(x) ⁺ beam current versus decaborane flow rate.

FIG. 14: Lifetime versus decaborane vapor flow rate.

FIG. 15: Etch rate of Si coupon.

FIG. 16: Ion implanter.

DETAILED DESCRIPTION Novel Ion Beam-Generating System

FIG. 1 shows an ion beam-generating system. As shown in this example, itis adapted to produce an ion beam for transport to an ion implantationchamber for implant into semiconductor wafers or flat-panel displays.Shown are ion source 400, extraction electrode 405, vacuum housing 410,voltage isolation bushing 415 of electrically insulative material,vacuum pumping system 420, vacuum housing isolation valve 425, reactivegas inlet 430, feed gas and vapor inlet 441, vapor source 445, feed gassource 450, reactive gas source 455, ion source high voltage powersupply 460, and resultant ion beam 475. An ion beam transport housing isindicated at 411. The ion source 400 is constructed to provide clusterions and molecular ions, for example the borohydride ions B₁₀H_(x) ⁺,B₁₀H_(x) ⁻, B₁₈H_(x) ⁺, and B₁₈H_(x) ⁻ or, or in addition, moreconventional ion beams such as P⁺, As⁺, B⁺, In⁺, Sb⁺, Si⁺, and Ge⁺. Ionsource 400 may be a Bernas-style arc-discharge ion source, which is mostcommonly used for ion implantation, or a “bucket”-type water-cooled ionsource which uses an immersed RF (radio frequency) antenna forming an RFfield to create ions, a microwave ion source, or an electron-impactionization source, for example. The gas and vapor inlet 441 for gaseousstate feed material to be ionized is connected to a suitable vaporsource 445, which may be in close proximity to gas and vapor inlet 441or may be located in a more remote location, such as in a gasdistribution box located elsewhere within a terminal enclosure. Aterminal enclosure is a metal box, not shown, which encloses the ionbeam generating system. It contains required facilities for the ionsource such as pumping systems, power distribution, gas distribution,and controls. When mass analysis is employed for selection of an ionspecies in the beam, the mass analyzing system may also be located inthe terminal enclosure.

In order to extract ions of a well-defined energy, the ion source 400 isheld at a high positive voltage (in the more common case where apositively-charged ion beam is generated), with respect to theextraction electrode 405 and vacuum housing 410, by high voltage powersupply 460. The extraction electrode 405 is disposed close to andaligned with the extraction aperture 504 of the ionization chamber. Itconsists of at least two aperture-containing electrode plates, aso-called suppression electrode 406 closest to ionization chamber 500,and a “ground” electrode 407. The suppression electrode 406 is biasednegative with respect to ground electrode 407 to reject or suppressunwanted electrons which otherwise would be attracted to thepositively-biased ion source 400 when generating positively-charged ionbeams. The ground electrode 407, vacuum housing 410, and terminalenclosure (not shown) are all at the so-called terminal potential, whichis at earth ground unless it is desirable to float the entire terminalabove ground, as is the case for certain implantation systems, forexample for medium-current ion implanters. The extraction electrode 405may be of the novel temperature-controlled metallic design, describedbelow.

(If a negatively charged ion beam is generated the ion source is held atan elevated negative voltage with other suitable changes, the terminalenclosure typically remaining at ground.)

Novel Actively Heated Extraction Electrode

The ion accelerating and ion beam forming effects (“ion optic effects”)of extraction electrodes are well understood by those skilled in thedesign of ion implantation systems.

Actively temperature-controlled extraction electrode designs are shownin FIGS. 9, 9B, 9C, 11 and 12, described later herein. An activelyheated extraction electrode arrangement of “sandwich” form, suitable forion beams of decaborane and octadecaborane, is shown in FIGS. 1A to 1L,which will be described now.

Referring to FIGS. 1A to 1L, extraction electrode 805 is comprised ofsuppression electrode element 810 and ground electrode element 820mounted in close succession along the ion beam path. The ions are drawnby electric field effects from the positively biased ion source 400,FIG. 1, to the extraction electrode 805. The ions propagate throughelectrode 805 along beam axis 530 as an energetic, focused, ribbon-formion beam 475. The ground electrode is maintained at the potential of thesurrounding vacuum housing 410 and establishes the potential of the ionbeam as it proceeds beyond the extraction electrode. Suppressionelectrode 810, biased to a few thousand volts negative relative to theground electrode, serves to suppress secondary electrons which aregenerated downstream from the suppression electrode due to beam strike.This prevents such energetic electrons from back-streaming into thepositively-biased ion source 400.

The suppression electrode element 810 and ground electrode element 820are fabricated of aluminum and have smooth, carefully polished surfacesto minimize local electric fields. The extraction optic component 805comprised of these elements is mounted on a manipulator 610A, shown inFIGS. 1E and 1F. This manipulator is used e.g. to align electrode 805with the ion source and downstream components and to vary the focallength of the ion optical system. As indicated, the manipulator enableslinear adjustment in the X dimension, transverse to the short dimensionof the slot-form aperture, and in the Z dimension, along the axis of theion beam. It also enables rotation about the X axis.

Each electrode element, 810 and 820, is comprised of two portions, inneraperture-defining portion, 810A and 820A, and disc-form outer portion,810B and 820B, respectively. Heater 830 is disposed (“sandwiched”)between these electrode elements, but is spaced out of contact with themso that heat transfer from heater to electrode elements is by radiation.Inner portions 810A and 820A form elongated, slot-form apertures A inthe electrode elements for passage of the ions from the ions source 400and serve to establish the electric fields to which the ions areexposed. Outer portions 810B and 820B of the electrode elements servemultiple functions: they support the inner electrode portions, theyserve as axially-directed, wide area heat receptors for absorbing heatthat radiates generally axially from the radiant heater 830 which isdisposed between them, and they define low-resistance thermal conductivepaths by which heat can flow by conduction radially from the outerportions to the inner portions. In the preferred form shown, eachelectrode element is of one piece, machined of aluminum, and as suchprovides excellent heat conducting paths from its outer to its innerelectrode portion. In other designs, the inner portions of theelectrodes may be discretely formed as replaceable units and may bethermally connected to permanently mounted outer portions by heatconductive metal gaskets compressed between the two portions. Also,instead of the outer portions of the electrode elements being planardiscs they may be of other heat receptive forms, such as of conical orof curved cross-section.

The radiant heater 830, mounted between the two outer electrode portions810A and 820A, is configured to surround the inner electrode portions810B and 820B and the ion beam path. In this implementation the heateris a circular tube heater, FIGS. 1A and 1F. Heater 830, of overalldiameter greater than the long dimension L of the slot-form ion beamapertures A, surrounds the apertures. It is centered on beam axis 530.Heater 830 is comprised of a hollow outer, chemically-resistantradiating tube 831, e.g. of stainless steel such as Incaloy™. Inside oftube 831 is centered an electric resistance heating element 832, e.g.nichrome wire. The resistance wire is held in its central position byinsulating material 833, for instance magnesium oxide, see FIG. 1J. Suchheaters are available for instance as Watrod™ heaters, from Watlow. Forprotecting the heater wire from chemical vapor during reactive gascleaning, glass hermetic end seals 835 are employed at the ends of theheater tube. Nickel plated steel end conductors 834 extend centrallythrough the seals, from the exterior to electrical connection with theresistance element 832 within the tube. With suitably chosen insulativestandoffs, e.g. of alumina (Al₂O₃), and by employing stainless steelconnectors, the entire extraction electrode unit is fluorine-resistant,and suitable for in situ cleaning by reactive gas.

An example of a suitable power control circuit for heater 830 is shownin FIG. 1L. Thermocouple 850 is connected with good thermal contact to athermally representative portion of the extraction electrode assembly805. Thermocouple 850 feeds back to a closed-loop PID controller, 860,e.g. Omron E5CK. Controller 860 is connected to solid state relay 865 ofpower circuit 870. In operation, the set point for heating theextraction electrode assembly is determined at the overall control unit880 for the ion beam producing system. The set point is fed to closedloop PID controller 860. Controller 860 interprets the set point signal,reads the temperature output of thermocouple 850 and controls the on andoff stages of relay 865 in manner to apply appropriate electric powerfrom power source 875 to heater 830 to achieve the desired temperatureat thermocouple 850.

As with the other embodiments described below, this heating arrangementis capable of maintaining the extraction electrode at a well-controlled,elevated temperature sufficiently high to prevent condensation ofdecaborane or octadecaborane vapor emanating from the relativelycool-operating ion source of FIGS. 7 and 7A, to be described. Theextraction electrode, made of fluorine-resistant materials, enablesperiodic in situ cleaning of the electrode to remove any deposits,employing fluorine vapors drawn through the extraction aperture of theassociated ion source 400. Such cleaning systems will now be described.

Reactive Gas Cleaning

FIG. 1 shows the reactive gas source 455 at terminal potential, withreactive gas inlet 430 incorporating a high voltage break 431, which canbe fabricated of an insulating ceramic such as Al₂O₃, for example. Sinceion sources for ion implantation can in general be biased up to amaximum voltage of about 90 kV, this high voltage break 431 must standoff 90 kV for that application. As will be described below, the cleaningsystem is used only with the ionizing source and high voltages off(de-energized), so that there is only high voltage across break 431 whenthe vacuum housing 410 is under high vacuum, which makes high voltagestandoff clearance requirements easier to meet. A dedicated endpointdetector 470, in communication with the vacuum housing 410, is used tomonitor the reactive gas products during chemical cleaning.

For ion sources suitable for use with ion implantation systems, e.g. fordoping semiconductor wafers, the ionization chamber is small, having avolume less than about 100 ml, has an internal surface area of less thanabout 200 cm², and is constructed to receive a flow of the reactive gas,e.g. atomic fluorine or a reactive fluorine-containing compound at aflow rate of less than about 200 Standard Liters Per Minute.

It is seen that the system of FIG. 1 enables in situ cleaning, i.e.without the ion source and extraction electrode being removed fromoperating position in the vacuum housing, and with little interruptionof service.

FIG. 2 illustrates another embodiment. The principal difference in FIG.2 over FIG. 1 is that the reactive gas source 455 and reactive gas inlet430 are at ion source potential. The benefits of this approach aretwofold: it is a more compact arrangement, and it allows the reactivegas source 455 and its associated gas supplies to be contained in thegas box which, at ion source potential, supplies gas and power to theion source 400, as is typical in commercial ion implantation systems.

Chemical Cleaning System

The embodiment of FIG. 3, having many features similar to FIG. 1, isconstructed to generate, selectively, both cluster ions and monomerions. It has a dedicated gas inlet 435 for feed material in normallygaseous state and is in communication, through valve 443, with a vaporsource 445 for producing borohydride and other vaporized feed materials.For conducting in-situ chemical cleaning of the ion source andelectrode, a remote plasma source 455 disassociates gas supplied by acleaning gas supply 465, for example NF₃, into decomposition productssuch as F, F₂, and N-containing compounds. When cleaning is desired,after de-energizing the ion source, the decomposition products are fedinto the ionization chamber from the outlet 456 of the remote plasmasource 455 by dedicated reactive gas inlet 430. The remote plasma source455 is mounted on the terminal potential side of voltage isolationbushing 415. Since the ion source 400 runs at high voltage, a highvoltage break 431 in vacuum provides voltage isolation.

To initiate a cleaning cycle, the ion source is shut down and vacuumhousing isolation valve 425 is closed; the high vacuum pump 421 of thevacuum pumping system 420 is isolated and the vacuum housing 410 is putinto a rough vacuum state of <1 Torr by the introduction of dry N₂ gaswhile the housing is actively pumped by backing pump 422. Once underrough vacuum, argon gas (from Ar gas source 466) is introduced into theplasma source 455 and the plasma source is energized by on-boardcircuitry which couples radio-frequency (RF) power into the plasmasource 455. Once a plasma discharge is initiated, Ar flow is reduced andthe F-containing cleaning gas feed 465, e.g. NF₃, is introduced intoplasma source 455. Reactive F gas, in neutral form, and otherby-products of disassociated cleaning gas feed 465, are introducedthrough reactive gas inlet 430 into the de-energized ionization chamber500 of ion source 400. The flow rates of Ar and NF₃ (for example) arehigh, between 0.1 SLM (Standard Liters per Minute) and a few SLM. Thus,up to about 1 SLM of reactive F as a dissociation product can beintroduced into the ion source 400 in this way. Because of the smallvolume and surface area of ionization chamber 500, this results in veryhigh etch rates for deposited materials. The ionization chamber 500 hasa front plate facing the extraction electrode, containing the extractionaperture 504 of cross sectional area between about 0.2 cm² and 2 cm²,through which, during energized operation, ions are extracted byextraction electrode 405. During cleaning, the reactive gas load isdrawn from ionization chamber 500 through the aperture 504 by vacuum ofhousing 410; from housing 410 the gas load is pumped by roughing pump422. Since the extraction electrode 405 (constructed, for instance, aselectrode 805 of FIG. 1A) is near and faces aperture 504 of ionizationchamber 500, the electrode surfaces intercept a considerable volume ofthe reactive gas flow. This results in an electrode cleaning action,removing deposits from the electrode surfaces, especially from the frontsurface of suppression electrode 406 (e.g. suppression electrode 810,FIG. 1A), which is in position to have received the largest deposits.Thus, it is beneficial to fabricate extraction electrode and itsmounting of F-resistant materials, such as Al (either aluminum oraluminum alloy) and the insulator elements of Al₂O₃.

The embodiment of FIG. 3 also has an endpoint detector consisting of adifferentially-pumped, Residual Gas Analyzer (RGA), constructed forcorrosive service. Analyzer RGA is in communication with vacuum housing410. It is to be used as a detector for the end point of the cleaningaction by monitoring partial pressures of F-containing reaction products(for example, BF₃ gas resulting from B combining with F). Other types ofendpoint detectors can be used, the RGA being shown to illustrate oneparticular embodiment. When the boron-containing partial pressuresdecline at RGA, the cleaning process is largely completed. Once thecleaning process is ended, the plasma source 455 is turned off and isbriefly purged with Ar gas (which also purges the ionization chamber500, the housing 410 and elements contained therein). The roughing pump422 is then isolated from direct communication with vacuum housing 410,the high vacuum pump 421 isolation valve is opened, and vacuum housing410 is restored to high vacuum (about 1×10⁻⁵ Torr or below). Then,vacuum housing isolation valve 425 is opened. The system is now ready toresume ion beam generation. The ion source voltage supply 460 can beenergized and ion source 400 operated normally.

An advantage of the embodiment of FIG. 3 is that the service facilitiesneeded to support the remote plasma source 455, such as cooling watercirculation and electrical power, can be at the terminal potential of anion implanter (see 208 in FIG. 16). This enables sharing facilitiesdenoted at S such as cooling water and electrical power, with themass-analyzer magnet 230 of the implanter. During cleaning mode, whenplasma source 455 is energized, the analyzer 230 is de-energized andtherefore does not need water or power, and vise versa, during ion beamproduction mode. This “sharing” can be accomplished by suitable controlarrangements represented diagrammatically at S′, which direct servicefacilities such as cooling water circulation and power supply connectionalternatively to the analyzer magnet 230, dashed arrow S, or to theremote plasma source 455, solid arrow S, depending upon the mode ofoperation being employed.

FIG. 4 shows an implementation similar to FIG. 2 for conducting in-situchemical cleaning of an source 400 and extraction electrode 405. Threeinlet passages are integrated into ion source 400, respectively forreactive gas 430 from plasma source 455, feed gas 435 from one of anumber of storage volumes 450 selected, and feed vapor 440 fromvaporizer 445. Unlike FIG. 3, the embodiment of FIG. 4 has theplasma-based reactive gas source 455 at the high voltage of ion source400. This enables the remote plasma source 455 to share control pointsof the ion source 400, and also enables the cleaning feed gas 465 andargon purge gas from storage 466 to be supplied from the ion source gasdistribution box, which is at source potential, see also FIGS. 6 and 6A.Also shown is a different type of endpoint detector, namely a FourierTransform Infrared (FTIR) optical spectrometer. This detector canfunction ex-situ (outside of the vacuum housing), through a quartzwindow. Instead, as shown in FIG. 4, an extractive type of FTIRspectrometer may be used, which directly samples the gas in the vacuumhousing 410 during cleaning. Also a temperature sensor TD may sense thetemperature of the de-energized ionization chamber by sensing athermally isolated, representative region of the surface of the chamber.The sensor TD can monitor heat produced by the exothermic reaction of Fwith the contaminating deposit, to serve as an end-point detection.

FIG. 5 shows an ion beam-generating system similar to that of FIG. 4,but incorporating a fundamentally different type of reactive gas source455. In this case, reactive ClF₃ gas contained in a gas cylinder is feddirectly into ion source 400 without use of a remote plasma source. Thispotentially reduces equipment cost and footprint since power andcontrols for a remote plasma source are not required. However, sinceClF₃ is pyrophoric, it is dangerous and requires special gas handling,whereas NF₃ (for example) is primarily an asphyxiant, and is less toxicthan many semiconductor gases, such as BF₃, PH₃, or AsH₃, and thereforesafer.

FIG. 6 shows plasma source 455, vapor source 445, source electronics,and service facilities S for the plasma source contained within a gasbox B meant for retrofit into an existing ion implanter installation.

The embodiment of FIG. 6 a differs from the embodiment of FIG. 6 above,by incorporating a preferred vaporizer and flow control system describedbelow. FIG. 6B is a valve schematic diagram for the ion source andself-cleaning system of FIG. 4.

Preferred Ion Source and Vaporizer

FIG. 7 is a diagram of a preferred ion source 10 and its variouscomponents, and see FIG. 7A. The details of its construction, as well asits preferred modes of operation, are similar to that disclosed byHorsky et al., International Application No. PCT/US03/20197, filed Jun.26, 2003: “An ion implantation device and a method of semiconductormanufacturing by the implantation of boron hydride cluster ions”, and byHorsky, U.S. patent application Ser. No. 10/183,768, “Electron impaction source”, filed Jun. 26, 2002, both herein incorporated by reference.The ion source 10 is one embodiment of a novel electron impactionization system. FIG. 7 is a cross-sectional schematic diagram of thesource construction which serves to clarify the functionality of thecomponents which make up the ion source 10. The ion source 10 is made tointerface to an evacuated vacuum chamber of an ion implanter by way of amounting flange 36. Thus, the portion of the ion source 10 to the rightof flange 36, shown in FIG. 7, is at high vacuum (pressure<1×10⁻⁴ Torr).Gaseous material is introduced into ionization chamber 44 in which thegas molecules are ionized by electron impact from electron beam 70,which enters the ionization chamber 44 through electron entranceaperture 71 such that electron beam 70′ is aligned with (i.e. extendsadjacent, parallel to) ion extraction aperture 81. Thus, ions arecreated adjacent to the ion extraction aperture 81, which appears as aslot in the ion extraction aperture plate 80. The ions are thenextracted and formed into an energetic ion beam 475 by an extractionelectrode 220 (FIGS. 8 and 9) located in front of the ion extractionaperture plate 80. Referring to FIG. 7, gases such as argon, phosphine,or arsine, for example, may be fed into the ionization chamber 44 via agas conduit 33. Solid feed materials 29 such as decaborane oroctadecaborane can be vaporized in vaporizer 28, and the vapor fed intothe ionization chamber 44 through vapor conduit 32 within the sourceblock 35. Typically, ionization chamber 44, ion extraction apertureplate 80, source block 35 (including vapor conduit 32), and vaporizerhousing 30 are all fabricated of aluminum. Solid feed material 29 isheld at a uniform temperature by closed-loop temperature control of thevaporizer housing 30. Sublimated vapor 50 which accumulates in a ballastvolume 31 feeds through conduit 39 and through throttling valve 100 andshutoff valve 110. The nominal pressure of vapor 50 between throttlingvalve 100 and shutoff valve 110 is monitored by heated pressure gauge60, preferably a capacitance manometer. Since the vapor 50 feeds intothe ionization chamber 44 through the vapor conduit 32, located in thesource block 35, and gases feed in through gas conduit 33, both gaseousand vaporized materials may be ionized by this ion source, which iscapable of creating ion beam 475 consisting of either molecular ions(such as B₁₈H_(x) ⁺) or monomer ions (such as As⁺), as needed. The ionsource may instead be a multi-mode ion source such as described in U.S.Pat. No. 7,022,999, issued Apr. 4, 2006, Entitled “Ion Implantation IonSource, System and Method”, or as described in U.S. patent applicationSer. No. 11/268,005, filed Nov. 7, 2005, entitled “Dual Mode ion Sourcefor Ion Implantation and Forming N-Type Regions with Phosphorus andArsenic Ions”, the contents of each of which, to the extent describingmulti-mode ion sources, being hereby incorporated by reference in theirentireties.

Vapor Flow Control into the Ion Generating System

The flow of vapor to ionization chamber of FIG. 7, and see FIG. 7B, isdetermined by the vapor pressure in the region just before vapor feedpassage 32, i.e., within shutoff valve 110 in FIG. 7. This is measuredby pressure gauge 60, e.g. a capacitance monometer, located betweenthrottling valve 100 and shut-off valve 110. In general, the flow rateis proportional to the vapor pressure. This allows the pressure signalto represent flow, and to be used as a set point to select flow. Togenerate a desired vapor flow into the ion source, vaporizer housing 30is brought to a temperature such that when throttling valve 100 is inits fully open position, the desired flow rate is exceeded. Then thethrottling valve 100 is adjusted to reach the desired pressure output.

To establish a stable flow over time, separate closed loop control ofthe vaporizer temperature and vapor pressure is implemented using dualPID controllers, such as the Omron E5CK control loop digital controller.The control (feedback) variables are thermocouple output fortemperature, and gauge output for pressure. The diagram of FIG. 7B showsa digital vapor feed controller 220 for performing these closed loopcontrol functions.

In FIG. 7B gauge output 250 from pressure gauge 60 is applied tothrottle valve position control 245 which applies throttle valveposition control signal 247 to throttle valve 100. Thermocouple output225 from vaporizer 28 is applied to vaporizer heater control 215 whichcontrols heater power 248 applied to the vaporizer 28.

A second, slow level of control is implemented by digital feedcontroller 220, accommodating the rate at which solid feed materialvaporizes being a function of its open surface area, particularly theavailable surface area at the solid-vacuum interface. As feed materialwithin the vaporizer is consumed over time, this available surface areasteadily decreases until the evolution rate of vapors cannot support thedesired vapor flow rate, resulting in a decrease in the vapor pressureupstream of the throttle valve 100. This is known as “evolution ratelimited” operation. So, with a fresh charge of feed material in thevaporizer, a vaporizer temperature of, say, 25 C might support therequired vapor flow at a nominal throttle valve position at the low endof its dynamic range (i.e., the throttling valve only partially open).Over time (for example, after 20% of the feed material is consumed), thevalve position would open further and further to maintain the desiredflow. When the throttle valve is near the high conductance limit of itsdynamic range (i.e., mostly open), this valve position is sensed by thecontroller 220, which sends a new, higher heater set point temperatureto the vaporizer heater control 215. The increment is selected torestore, once the vaporizer temperature settles to its new value, thenominal throttle valve operating point near the low end of its dynamicrange. Thus, the ability of the digital controller 220 to accommodateboth short-timescale changes in set point vapor pressure andlong-timescale changes in vaporizer temperature makes the control ofvapor flow over the lifetime of the feed material charge very robust.Such control prevents over-feeding of vapor to the ionization chamber.This has the effect of limiting the amount of unwanted deposits onsurfaces of the ion generating system, thus extending the ion sourcelife between cleanings.

FIG. 8 shows a top view (looking down) of an ion extraction electrode220 facing the novel ion source 10. The ion source 10 is held at apositive potential V_(A) with respect to the ion extraction electrode220, which is at local ground potential, i.e., at the potential of thevacuum housing. The ion extraction electrode 220 is a simple diode;electrode plate 302 is the “ground” electrode and plate 300 the“suppression” electrode, typically held a few thousand volts belowground potential by suppression power supply V_(S). The ionizationchamber 44 and ion extraction aperture plate 80 of ion source 10 areshown facing extraction electrode 220. The three plates 80, 300, 302contain rectangular slots or apertures through which ions 90 areextracted; FIG. 8 illustrates the slot profiles in the “short”, ordispersive, direction.

Further Embodiments of Novel Heated Electrode

During the decaborane lifetime tests shown in FIG. 14, a novel heatedaluminum electrode was used. FIG. 9 shows a top view of the basicoptical design of the extraction system, in the dispersive plane of theone-dimensional “slot” aperture lenses. In the implanter used, theionization chamber 490 of the ion source was held at the desired ionbeam energy by positive high voltage power supply V_(A), FIG. 8. Forexample, if a 20 keV ion beam is desired, then V_(A)=20 kV. Ionextraction aperture plate 500 is electrically isolated from ionizationchamber 490 such that it can be biased by bipolar power supply V_(B)from −750V-750V. The isolation is accomplished by a thermallyconductive, electrically insulating polymeric gasket which is sandwichedbetween the ion extraction aperture plate 500 and ionization chamber490. The parts of the ion source body that are exposed to vapor (sourceblock 35, ionization chamber 44, and extraction aperture plate 80 inFIG. 7) are maintained in good thermal contact with each other tomaintain controlled temperature surfaces during source operation. Ionsproduced in ionization chamber 490 are extracted through the aperture inion extraction aperture plate 500 by extraction electrode 540 consistingof suppression electrode 510 and ground electrode 520. The ionspropagate as a focused ion beam along the beam axis 530. Suppressionelectrode 510, biased to a few thousand volts negative by power supplyV_(S), serves to suppress secondary electrons which are generateddownstream from the suppression electrode due to beam strike, preventingthese energetic electrons from back streaming into the positively-biasedion source. The ionization chamber 490, ion extraction aperture plate500, suppression electrode 510, and ground electrode 520 are allfabricated of aluminum, and have smooth, carefully polished surfaces tominimize local electric fields.

An important effect of biasing ion extraction aperture plate 500 is tochange the focal length of the ion optical system of FIG. 9. A negativebias increases the focal length, while a positive bias decreases thefocal length. For large biases, the effect can be substantial. Fordiagnostic purposes, a scanning-wire profilometer was installed, locatedat the entrance to the analyzer magnet, just downstream of the sourcehousing isolation valve (210 in FIG. 16). This scanner recorded the beamcurrent distribution in the dispersive plane, useful to determine howwell the ion beam is being focused in the dispersive plane. 20 keVoctadecaborane beam profiles are shown in FIG. 9 a for three differentbiasing conditions: −483V, 0, and +300V. The zero volt condition issubstantially over focused, the positive voltage condition more overfocused, and the negative voltage condition properly focused. Theelectrode position was held constant during the three measurements. Asexpected, the proper focusing condition yielded the highest ion beamcurrents.

The ability to change the optical focal length, and thus tune theoptical system to obtain the highest ion beam current, enablesintroduction of the least amount of feed material to the vaporizer.Again, this has the beneficial effect of limiting the amount of unwanteddeposits on surfaces of the ion generating system, extending the ionsource life between cleanings.

Besides the biasing of the extraction aperture plate for focusing thesystem just described, the invention provides means for moving theextraction electrode optic element relative to other components of thesystem. FIG. 10 shows the novel electrode 600 mounted on a three-axismanipulator 610 which allows for motion (with respect to the ion source)in X, Z and Θ, as defined by coordinate system 620. Actuator 613controls X-motion, actuator 612 controls Z-motion, and actuator 611controls Θ-motion. The manipulator 610 mounts to the side of theimplanter vacuum housing via mounting flange 615.

FIG. 11 shows a partial exploded view of the radiatively-heated versionof the novel electrode head. Shown are suppression electrode 700, groundelectrode 710, heater plate 720, and radiant heater wire 730. Thesuppression and ground electrodes are fabricated of aluminum, the heaterplate of stainless steel, and the heater wire 730 of nichrome. When theelectrode was operated at 200 C, power consumption was about 60 W tomaintain the temperature. The heater power is controlled with aclosed-loop PID controller, the Omron E5CK, based on read back of athermocouple.

FIG. 12 shows a partial exploded view of a resistively-heated version ofthe novel electrode head. Shown are suppression electrode 800, groundelectrode 810, and resistive heaters 820. The four resistive heaters 820fit into sleeves 830, two into each electrode plate. The sleeves 830 area split design such that the heater press-fits into the sleeve,achieving intimate contact. Intimate contact between heater andelectrode is important to insure proper heating of the electrode, and toprevent premature burnout of the heaters. Again, the Omron E5CK orequivalent can control the electrode temperature based on read back of athermocouple.

As described above, use of these heating arrangements for the extractionelectrode maintain a well-controlled, elevated temperature sufficientlyhigh to prevent condensation of decaborane and octadecaborane such asproduced by the relatively cool-operating ion source of FIGS. 7 and 7A.The extraction electrode made of fluorine-resistant materials, e.g.aluminum; enables periodic in situ cleaning of the electrode to removeany deposits by fluorine vapors drawn through the extraction aperture.

A different situation is encountered with plasma ion sources thatinherently run so hot that the heat may harm the extraction electrodeassembly if made of low temperature material. Referring to FIG. 9, shownin dotted lines are circular cooling coils, 512 and 522 secured in heattransfer relationship to the backs of aluminum electrode members 510 and520, respectively. Circulation of cooling fluid through these coolingcoils can cool the aluminum electrodes to prevent deformation by heatfrom hot ion sources. This enables use of fluorine-resistant materialsfor the extraction electrode, for instance aluminum or a complexcontaining aluminum, which provide resistance to attack by any fluorinepresent from feed materials or from reactive cleaning gas.

Referring to FIG. 9B, a temperature-controlled extraction electrode 540Ais provided for use with a multimode ion source 490A capable ofoperating, alternatively, at cool and high temperature modes, or withreplaceable units that operate at respective low and high temperaturemodes. The extraction electrode assembly 540A, formed for instance ofaluminum, is equipped for active heating and cooling.

In cool ion source mode, useful with vapors of decaborane andoctadecaborane, the extraction electrode is actively heated to deterformation of deposits on the electrode surfaces. The ion source may forinstance operate by “soft” electron impact. In this case it employs afocused electron beam 70 as described above in relation to FIGS. 7 and7A.

At the high temperature mode, the extraction electrode is activelycooled to enable it to be formed of relatively low temperature materialsuch as aluminum. For a hot mode ion source, for instance, a hot plasmamay be maintained by an arc-discharge within the ionization chamber,produced by an electron-emitting cathode and negatively biased electronrepeller disposed within a confining magnetic field. Principles ofdesign and construction of arc-discharge plasma ion sources, per se, arewell known, see for instance Freeman, U.S. Pat. No. 4,017,403 andRobinson, U.S. Pat. No. 4,258,266, incorporated herein by reference inthis respect. The arc discharge creates a relatively hot plasma whichionizes gas introduced to the ionization chamber. In both hot andcool-operating ion source modes the beam produced may be of ribbonshape, its elongated cross-section produced by the similarly elongatedshape of the extraction aperture of the ion source and the aperturethrough the extraction electrode.

As shown in FIG. 9B, circular, radiant tube heater 550, such as thetubular heater described with reference to FIGS. 1A-1K, is arrangedcoaxially with the ribbon-shaped path 530 of ions from the ion source.Heater 550 is of diameter larger than the long dimension of theslot-form aperture through the electrode elements. The tubular heater ismounted downstream of the suppression electrode element in position todirectly radiate heat to outer structure of both the extractionelectrode element 520 at ground potential and the suppression electrodeelement 510 at relatively negative potential. The suppression electrodeelement is of the form described above, with an outer disc-shapedportion defining a broad, axially-directed face area adapted to receiveand absorb heat radiating from the heater 550, and to conduct the heatradially to the inner portion of the electrode element. The groundelectrode element is of smaller radial extent, and is constructed to beheated principally by heat radiating radially-inwardly from thesurrounding heater.

By suitable temperature sensing, as by the thermocouple of FIG. 1L, arepresentative temperature of the extraction electrode assembly isdetected. By use of a suitable controller, electric heating currentflows through the internal resistive element of the tubular heater tomaintain the temperature of the thermocouple at a set point. As with thecontroller shown in FIG. 1L, the set point is selected to preventcondensation of feed vapors that reach the extraction electrode. Forinstance, when ionizing decaborane and octadecaborane, the temperatureof the extraction electrode may be maintained at about 150 C by activeheating by the heater.

The extraction electrode assembly of FIG. 9B is also equipped withcooling coils, 512 and 522 that surrounds the beam path. The coils aresecured in conductive heat transfer relationship to the backs of thealuminum electrode elements 510 and 522. As described regarding FIG. 9,circulation of cooling fluid through the cooling coils, cooling thealuminum electrode elements, can prevent electrode deformation by heatfrom a hot ion source, that otherwise might disturb the electric fieldsproduced by the electrode elements.

When it is desired to employ the multimode ion source apparatus in a hotmode to produce ion beams of suitable species, the walls of theionization chamber are permitted to operate at a substantially elevatedtemperature, e.g. above 400 C. In this mode of operation, heating of theextraction electrode assembly is disabled and a flow of cooling liquidis maintained in coils 512 and 522 to cool the aluminum electrodeelements below a temperature at which detrimental distortion of theelectrodes might occur, i.e. below about 400 C.

This electrode assembly combined with the multimode ion source issuitable for use in systems having in-situ reactive gas cleaning,described in relation to FIGS. 1 to 6B and 7B. The aluminum compositionof the electrode elements and the corrosive-resistant sheath of thetubular heater are resistant to attack from fluorine.

Referring to FIG. 9C, the extraction assembly may instead comprise threeelectrode elements: suppression electrode 510, as in FIG. 9B, a centralelectrode 515, and the main extraction electrode 520′. Electrode 520′ ismaintained at ground (housing) potential, the potential of centralelectrode 515 is variable and can be maintained at a selected potentialbetween ground and a considerable value, for instance −30 KeV. Thepotential of suppression electrode 510 may float at voltage Vs, forinstance, −10 KeV, relative to the central electrode. In thisarrangement the ion-accelerating and focal-length adjusting effects ofthe electrode system may be varied for obtaining desired effects uponthe ion beam.

The electrode elements are nested, as shown in FIG. 9C, so that the beampassing through the electrode elements is exposed only to the respectivepotentials of the three elements. In the arrangement shown, the twoouter electrode elements are supported by circular radiantheat-receiving, heat-conductive disc portions, while the centralelectrode is of less transverse extent, supported between the outerelectrodes by suitable support structure, not shown. A tubular heater560 of suitable dimension is disposed between the two outer electrodestructures 510, 520′, and surrounds inner electrode structure 515. Theheater is constructed and arranged to heat the three electrodestructures by direct radiation to each. Radiation proceeds generallyaxially toward the outer disc portions of the outer two electrodeelements, and radially inwardly toward the center electrode. By suitabletemperature sensing, as by the thermocouple previously mentioned, arepresentative temperature of the electrode assembly is detected.Electrical heating current through the tubular heater maintains thetemperature at a set point to prevent condensation on the electrodes offeed vapors that reach the assembly. For instance, when ionizingdecaborane and octadecaborane by “soft” electron impact, the temperatureof the electrode elements may be maintained at about 150 C. Similar tothe embodiment of FIG. 9B, each of the electrode structures is alsoprovided with a cooling coil 512 and 522, and the assembly of FIG. 9Cmay instead be cooled as described when an ion source operates in hotmode.

While the heated arrangement of a three-electrode system has been shownin FIG. 9C with respect to a multimode ion source, it is of courseuseful with replaceable hot and cool mode ion sources. For use with onlycool-operating ion sources, such as shown in FIGS. 7 and 7A, the coolingfeature may be omitted from the electrode assembly, or the coolingfeature may be retained, available for use with an arc-discharge hotplasma source that may, from time to time, be substituted for the softelectron impact source.

Source Lifetime Measurements when Running Decaborane

FIG. 14 shows the results of source lifetime testing over a broad rangeof decaborane flows. The fit to these data is from Equation (3). Nofailures of the ion source were recorded during these tests; rather, theindividual tests were ended when the decaborane ion current dropped toroughly half of its initial level. Upon inspecting the ion source, itwas found that a substantial amount of boron-containing material wasdeposited within the ionization chamber, mostly adhering to the interiorwalls of the chamber. In some cases, the ion extraction aperture wasalso partially occluded. The model of Equation (3) seems to fit the datawell, and suggests that “lean” operation is the key to prolonged ionsource lifetime, between in situ chemical cleaning procedures ordisassembly.

Measurements of Etch Rates within Ionization Chamber During F Cleaning

The system with the ion source 10 of FIG. 7 was used to test the Fcleaning process on 1-mm-thick silicon coupons staged inside of theionization chamber 44, with the following modification: rather thanincorporating a dedicated reactive feed conduit, the vapor feed conduit32 was employed to introduce the reactive gas. Si was used becauseetching of Si by F is well understood, and pure Si material is availablein the form of Si wafers. This test required removing the vaporizerbetween cleaning cycles. Two coupon locations were tested: one havingline-of-sight relationship with the reactive gas inlet (i.e., the vaporfeed 32), and one not having line-of-sight. The etch rates are shown inFIG. 15 as a function of NF₃ flow rate. During this process, a flow of700 sccm of argon was maintained into the remote plasma source while theNF₃ flow rate was varied from 50 sccm to 500 sccm. A line-of-sightgeometry shows a factor of about five increase in etch rate, and istherefore a preferred geometry if it can be done uniformly. To this end,the geometry portrayed in FIG. 3 should provide better etch uniformityof the ion source ionization chamber 44 than the geometry shown in FIG.4. The test also indicated that location of etch-sensitive componentsshielded from the gas flow is effective to provide a degree ofprotection to those components.

To extend the life of components of the self-cleaning ion generatingsystem construction materials are selected that are resistive to thereactive gas, and provision can be made for shielding of sensitivecomponents.

For the interior of the ionization chamber, as indicated above, aluminumis employed where the temperature of the ionizing action permits becausealuminum components can withstand the reactive gas fluorine. Wherehigher temperature ionizing operation is desired, an aluminum-siliconcarbide (AlSiC) alloy is a good choice for the surfaces of theionization chamber or for the extraction electrode. Other materials forsurfaces in the ionization chamber are titanium boride (TiB₂), BoronCarbide (B₄C) and silicon carbide (SiC).

For components exposed to the fluorine but not exposed to the ionizingaction, for instance electron source components such as electrodes, thecomponents may be fabricated of Hastelloy, fluorine-resistant stainlesssteels and nickel plated metals, for instance nickel-plated molybdenum.

Both inert gas shields and movable physical barriers can protectcomponents of the system from the reactive gas during cleaning. Forexample, referring to FIG. 7A, a conduit 113 for inert gas, for instanceargon, extends from a gas source, not shown. Its outlet is at astrategic location in the ion source, such that flow of the inert gas,when initiated for the cleaning cycle, floods the component to beprotected. In FIG. 7A the outlet 113 a of inert gas conduit 113 aims aflooding stream of argon over the active components of electron gun 112,including, the electron-emitting cathode. In FIG. 7A a movable shieldmember 73 is also shown, which is movable into position for the cleaningcycle. In the example shown, it is movable over the aperture 71A leadingto beam dump 72, or to another electron gun when provided on that sideof the ionization chamber 44.

The cleaning process described above was conducted to observe its effecton boron deposits within the ionization chamber and on the interior ofthe ion extraction aperture of the novel ion source 10 of FIG. 7. Theobserved etch rates had characteristics similar to the plot of FIG. 15,but were a factor of 3 lower. Thus, for a NF₃ flow rate of 500 sccm, theetch rate for decaborane deposits were 7 μm/min (no line-of-sight), and36 μm/min (line-of-sight). The interior of the ion extraction apertureafter running 4 hrs of decaborane at 0.8 sccm vapor flow had about 133μm thick boron-containing deposit prior to cleaning. Observations weremade after a 5 min F clean, and after a 15 min F clean using these flowrates. One side of the aperture plate was in line-of-sight with thevapor feed. It was observed from the cleaning pattern that the vaporfeed aperture is centered in the vertical direction! After 15 minutes ofcleaning, the plate was almost completely free of deposits. Also, thenovel heated aluminum ion extraction electrode of FIG. 10 was removedand inspected after long operation. It was very clean with no observabledecaborane deposits. This was undoubtedly due to exposure of theelectrode to reactive F (F can flow through the ion source ionextraction aperture located in front of the vapor conduit, to theextraction electrode directly in front of it). Also, elevatedtemperature of the Al electrode assembly increased the effective etchrate of its deposits.

With respect to the ionization chamber, again, a 15 min etch clean leftthe chamber nearly free of deposits. A test was conducted in which thesystem was repeatedly cycled in the following manner: two hours ofdecaborane operation (>500 μA of analyzed beam current), the source wasturned off and the filament allowed to cool, followed by a 15 minchemical clean at 500 sccm of NF₃ feed gas and 700 sccm of Ar, to see ifconducting repeated chemical cleaning steps was injurious to the ionsource or extraction electrode in any way. After 21 cycles there was nomeasurable change in the operating characteristics of the ion source orthe electrode. This result demonstrates that this F cleaning processenables very long lifetime in ion source operation of condensablespecies.

The Ion Generating System Incorporated into an Exemplary Ion Implanter

FIG. 16 shows the basic elements of a commercial ion implanter, with anembodiment of the novel ion beam generation system incorporating the ionsource of FIG. 7 installed. The ion source 10 is inserted into thesource vacuum housing 209 of the ion implanter. It is electricallyinsulated from housing 209 by insulator 211. The ion extractionelectrode 220 extracts and accelerates ions from the ion source 10 toform an ion beam 200. Ion beam 200 propagates entirely in vacuum; fromthe electrode 220 it enters analyzer housing 290, 300 where it is bentand dispersed by dipole analyzer magnet 230 into separate beamlets whichdiffer by their charge-to-mass ratio. The ion beamlet of interest passesthrough mass resolving aperture 270 and into a final acceleration (ordeceleration) stage 310. The thus-produced,-selected and-accelerated ionbeam 240 leaves the ion beam forming system 208 and is introduced to theprocess chamber 330 where it intercepts one or more device wafers 312 onrotating disk 314. The ion source vacuum housing 209 can be isolatedfrom the remainder of the implanter's vacuum system by closing isolationvalve 210. For example, isolation valve 210 is closed prior to in situcleaning of the ion source and the ion extraction electrode 220. Theextraction electrode 220, may be temperature controlled in any of theways described above.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

1. An ion extraction electrode assembly constructed for use in an ionimplantation system having an ion source that has a low temperatureoperating mode, the ion extraction electrode constructed and adapted toextract ions from the ion source as an ion beam for transport to asurface for ion implantation, the ion extraction electrode combined witha controllable heater constructed to maintain the extraction electrodeat an elevated temperature to counter condensation on the electrode ofgases or vapors leaving the ion source forming an assembly, the assemblycomprised of aluminum and constructed also for use in an ionimplantation system having an ion source that has a relatively hightemperature operating mode, the ion extraction electrode combined with acooling device for operation during high temperature operation of theion source, while the heater is de-energized, to cool the electrode to atemperature below about 400 C.
 2. An ion extraction electrode assemblyconstructed for use in an ion implantation system having an ion sourcethat has a low temperature operating mode, the ion extraction electrodeconstructed and adapted to extract ions from the ion source as an ionbeam for transport to a surface for ion implantation, the ion extractionelectrode combined with a controllable heater constructed to maintainthe extraction electrode at an elevated temperature to countercondensation on the electrode of gases or vapors leaving the ion sourceforming an assembly, the assembly constructed for extracting positivelycharged ions from an ion source and comprising at least two electrodeelements in succession along an ion beam path through the electrodeelements, the electrode element constructed to be nearest the ion sourcecomprising a suppression electrode adapted to be maintained at a voltagemore negative than the succeeding electrode element to suppress electronmovement toward the ion source.
 3. An ion extraction electrode assemblyconstructed for use in an ion implantation system having an ion sourcethat has a low temperature operating mode, the ion extraction electrodeconstructed and adapted to extract ions from the ion source as an ionbeam for transport to a surface for ion implantation, the ion extractionelectrode combined with a controllable heater constructed to maintainthe extraction electrode at an elevated temperature to countercondensation on the electrode of gases or vapors leaving the ion sourceforming an assembly, in which there are two electrode elements, eachhaving a heat-receptive surface for absorbing heat, the heatercomprising a heat radiator located between the heat-receptive surfacesof the two electrode elements in position to heat each by radiation andin which the heat-receptive surfaces face generally in the direction ofextent of a beam path through the electrode and the heater is spaced inthe direction of extent of the beam path from the heat-receptive surfaceand in which at least one electrode element of the extraction electrodecomprises an inner portion defining a beam aperture and an outer portionin heat-conductive relation to the inner portion, the outer portiondefining a heat-receptive surface for absorbing heat radiated from theheater.
 4. The ion extraction electrode assembly of claim 3 in which theheat-receptive surface of each of the elements is of disc form and theheater comprises a tubular heater unit formed to substantially surroundthe beam path.
 5. The ion extraction electrode assembly of claim 3having a third electrode element disposed between the aforementionedelectrode elements, the third electrode element of the extractionelectrode comprising a portion that both defines a beam aperture and aheat-receptive surface for absorbing heat radiated from the heater.